1. Field of the Invention
The present invention generally relates to a composite nano-particle in which each of more than two kinds of chemical species has a phase individually and forms a nano-sized particle and method for preparing the same and, more particularly relates to a nano-sized nano-particle for, for example a composite low energy electron exciting light emitting phosphor etc., composed of independent good dispersible particles, which is formed without coagulation of composite nano-particles in the process of production.
2. Discussion of Background
In recent years, it has been noted that a nano-crystal (nano-structure crystal) has unique optical characteristics in an ultrafine particle such as Si, Ge, etc. porous silicon, semiconductor of II-IV group element in the periodic table. The nano-structure crystal herein used means a crystal particle having a particle diameter of several nm (nano-meters) and often referred to as nano-crystal.
By the way, different types of phosphors are used in a display such as a television etc. A phosphor currently used for a display of a television etc. is synthesized by firing raw materials at high temperatures. The particle diameter of the phosphor thus synthesized is several micrometers (3-10 μm). On the other hand, in recent years, a low-profile display has been desired in the filed of television etc. and a lightweight flat display such as a plasma display (hereinafter referred to as PDP), field-emission-display (hereinafter referred to as FDD), electro-luminescence-display (hereinafter referred to as ELD) has received considerable attention.
When the above-described FED which has received particular attention is formed in low-profile, it is necessary to lower the voltage of electron beam. However, when such a phosphor having a particle diameter of several micrometers as above described is used in a low-profile display, sufficient emission can not be obtained because low voltage of electron beam. That is to say, such a low-profile display can not excite sufficiently a conventional phosphor.
A low energy electron exciting light emitting phosphor is generally used in a vacuum fluorescent display. Particularly, a nano-sized phosphor is suitable for an FED, a highly-refined thin-profile vacuum fluorescent display, etc.
On the other hand, a nano-crystal phosphor can excite the afore-mentioned phosphor with an electron beam irradiated by low voltage and to emit light. A phosphor which satisfies such requirements is exemplified by a II-IV group semiconductor having such a nano-structure crystal as above described.
Unexamined Patent Publication (Kokai) No. 10-310770 discloses a method of preparing a nano-sized phosphor by using co-precipitation. In such a conventional invention as disclosed in the publication, a nano-sized phosphor particle doped with an activating agent by a liquid phase reaction using co-precipitation is formed and simultaneously, an organic acid acrylic acid, methacrylic acid, etc. is added to the liquid phase reaction. Thereby, the surface of the afore-mentioned phosphor particle is coated with the organic acid and defects on the surface of the phosphor particle are decreased to make improvements in the emission efficiency possible.
Further, an activating agent is dispersed uniformly on a II-IV group semiconductor by the afore-mentioned liquid phase reaction using the co-precipitation method. According to such a prior art, the only organic acid is boned to ZnS:Mn via S—O bond added. Unexamined Patent Publication (Kokai) No. 10-310770 describes on pages 6 and 8 that when ZnS:Mn is bonded with an organic acid, it coats the surface of the phosphor particle and supplies energy required for light emission. Since, however, coating with only organic acid is performed is such a technique as described in Unexamined Patent Publication (Kokai) No. 10-310770, there is a problem in the stabilization.
Unexamined Patent Publication (Kokai) No. 2000-265166 discloses a phosphor which is formed by coating a nano-sized nano-crystal phosphor particle ZnS-Tb obtained by the liquid phase reaction using precipitation with a glass component. Such a glass component as described in the publication is a gel-like (—SiO—) n film obtained by polymerizing tetrakissilane in ethanol, ion-exchanged water and hydrochloric acid.
In such an invention as described in Unexamined Patent Publication (Kokai) No. 2000-265166, a nano-crystal phosphor or another composite nano-particle obtained by the liquid phase reaction using precipitation is reacted with a gel-like glass component, that is, coated directly with a gel-like (—SiO—) n film obtained by polymerizing tetrakissilane in ethanol, ion-exchanged water and hydrochloric acid, thereby improving the light emission efficiency in electron beam excitation emission.
However, a composite nano-particle the surface of which is coated with a glass component constituted by such a prior art aggregates, similarly to a conventional composite nano-particle. Therefore, original characteristics of nano-crystal can be hardly obtained.
Among the afore-mentioned nano-crystal, a semiconductor nano-crystal having a quantum size effect is situated in a transition region between a bulk semiconductor of mono-crystal or large particles and a molecule and shows characteristics different from them. Therefore, it has received considerable attention in a variety of fields.
In the case of a nano-crystal having specific characteristics for a crystal particle of a metal or insulator having a particle diameter of several nano-meters other than the above-described semiconductor nano-crystal, nano-crystal phosphor, a nano-crystal having excellent dispersibility is provided.
Tremendous research effort has been made on the optical characteristics of the afore-mentioned semiconductor nano-crystal toward the practical use for an optical material, phosphor material, photo-catalyst, etc. However, such a semiconductor nano-crystal has problems to be solved, that is to say, it is unstable compared with a bulk semiconductor and easy to be decomposed and degraded, control of particle diameter is hard because of the presence of particle distribution, synthetic method is hard or complicated, mass-production is hard, etc.
By the way, among the afore-mentioned semiconductor nano-crystals, a method of preparing a conventional nano-sized phosphor has such defects as described below. While a nano-sized phosphor particle can be easily synthesized by a general co-precipitation method of a prior art, nano-sized phosphor particles aggregate each other immediately in a reaction solution and the particle size of the nano-sized phosphor particle thus obtained is made apparently larger than Bohr diameter of ZnS. Therefore, there are problems that a group of thus aggregated phosphor particles is coated with an organic acid or glass component, and particle size is made larger furthermore, and the quantum size effect or quantum confined effect can not be sufficiently exerted.
Next, the afore-mentioned quantum size effect and quantum confined effect are explained in detail. Change in physical properties depending on size appears particularly in optical properties. Specifically, when the particle size of the nano-sized phosphor particle is made smaller than the effective Bohr diameter of ZnS, it is well known that its electron state is different from that of bulk semiconductor. That is to say, when a phosphor is a very small atomic group in the order of nano-meter, a band rank which must be continuous originally is discontinuous with an increase in number of atom, and consequently the energy of HOMO lowers, and the energy of LUMO rises. Therefore, a band gap increases and excitation energy also increases. These phenomena are called “quantum size effect.”
When a phosphor absorbs an electron beam or light, it forms an electron and a hole in a band. While an electron and a hole move individually in a state of bulk, a pair of electron-hole (exciton) is formed stably because the electron and hole are confined in a very narrow space in a nano-particle. Thereby, the energy transportation efficiency to light emission, that is, the light emission quantum efficiency increases. These phenomena are called “quantum confined effect.”
A combination of chemical species constituting a nano-crystal must be selected, depending on a specific objective. Such a combination is exemplified by [metal/oxide/sulfide] or [conductor/semiconductor/insulator]. For example, CdS/SiO2, CdS/CdSe, Au/SiO2, etc. are exemplified.